1. Field of the Invention
The present invention generally relates to image sensing systems and more particularly relates to a self-calibration method and circuit architecture of linear image sensors that can be used in scanners, facsimile, photocopy machines and other image reproduction systems.
2. Description of the Related Art
There are many applications that need an imaging system to convert a target to an electronic format that can be subsequently analyzed, printed, distributed or archived. The electronic format is generally a digital image of the target. A typical example of the imaging system is a scanner and the target is a sheet of paper from a book or an article. Through the scanner, an electronic or digital image of the paper is generated and subsequently may be analyzed, computed, or transmitted through the Internet.
An imaging system generally includes an image sensing module that converts a target optically into an image. The key element in the sensing module that converts the target optically to the image is an image sensor comprising an array of photodetectors responsive to light impinged upon the image sensor. Each of the photodetectors produces an electronic signal representing the intensity of light reflected from the target. The electronic signals from all the photodetectors are readout as a video signal that is then digitized through an analog-to-digital converter to produce a digital signal or an image of the target.
FIG. 1A illustrates a configuration system 100 that has been used for the past tens of years. A scanning document 110 that can be a page from an article or book is scanned in by an image sensing system 111 that can be a scanner, such as SCANJET 4100CSE Color Scanner from Hewlett Packard. The output of the scanner is typically a digital image 114 of scanning document 110. Scanner 111 includes an image sensor 112 and an analog-to-digital converter 115. Image sensor 112 generates images 117 that are typically digitized to gray scale or color images of 8-bit precision. Binalization process 116 receives and converts image 114 to binary image 118 that is a preferred form for data analysis and understanding in data process 120. Binalization process 116 is typically implemented in a separate circuit or a software application. The separate circuit may be implemented in a post-processing circuit coupled to A/D converter 115 and the software application may be embedded in a scanner driver or provided in a commercial image editing software, such as Adobe PhotoShop, running in a host computer 119.
FIG. 1B depicts a contact image sensor (CIS) system that can be used in image sensor 112 of FIG. 1A. Scanning document 110 is illuminated by an illumination source 121. Reflected light from scanning document 110 is collected and focused by a full-width rod-lens system 122. The CIS system allows one-to-one scanning of the document because rod lens 122 and an image sensor chip 124 are of the same width as (or greater width than) scanning document 110.
FIG. 1C is a functional block diagram of image sensor 112, along with FIG. 1D showing some detail of the construction of image sensor array 126. To be specific, a plurality of individual sensor chips 130 are butted end-to-end on a single substrate. Each of the individual sensor chips comprises a plurality of photodetectors 128 arranged in a row. In operation, image sensor array 126 is triggered by a start pulse to the first-in-sequence individual sensor chip 130 which serially activates the photodetectors on the first individual sensor chip 130. After the signal from the last photodetector element of the first individual sensor chip 130 is read, an end-of-scan pulse is generated so that the next sensor chip in sequence is triggered.
The number of individual sensor chips chosen is dependent upon the desired width of scanning. Sensor array 126 also comprises necessary circuits to serially activate the individual chips and to readout signals generated from photodetectors. The strength of the signals is directly proportionate to the reflected light from the scanning document. To preserve the contents in the scanning document, most CIS systems produce signals that are subsequently digitized to 8 or 12 bit data by a following analog-to digital (A/D) converter.
In many imaging applications, such as check verification at checkout counters in a retail store and document archival, the primary interest is to extract texture information from captured images, for example, for optical character recognition (OCR). To be applicable for such process, the images are preferably in binary format, namely the texture information in black and the background in white or vice versa. In other words, the digitized signals from the A/D converter must be binalized.
The photodetectors in sensor array 126 are, however, subject to several inherent shortcomings that may cause sensor array 126 to produce errors that are hardly correctable in binary data. One of the shortcomings is that the gain from the photodetectors is not uniform from one photodetector to another within a chip or within the array. For example, the base of an NPN photodetector is formed by ion implantation. There is typically a .+-.5% non-uniformity across a wafer subject to ion implantation. This non-uniformity results in a current gain variation of as much as .+-.30% across the wafer. The non-uniformity of the gain yields a non-uniformity of the photo response of the same magnitude. Another shortcoming that may adversely affect the performance of sensor array 126 is inherent noises from some or all of the photodetectors or the non-uniformity of the wafer. Signals generated by the photodetectors could be distorted by the noises, which could cause misrepresentation of the contents in scanning objects. There is therefore a great need for an image sensor that produces signals, ideally, independent from the noises or at least with minimized noise effects.